Fluorine-containing resins (plastics) have been used in a large amount for important parts such as chemical pipe, square tank and basket for silicon wafer which have a direct influence on semiconductor products in the semiconductor production process where strict requirements for cleanliness are demanded. Therefore reduction of contents of metal components, TOC (total organic carbon) and particle impurity in the fluorine-containing resins has been attempted positively.
Also with respect to fluorine-containing elastomers comprising mainly a vinylidene fluoride (VdF) unit or tetrafluoroethylene (TFE) unit, as they are called fluorine-containing rubbers, they are used in the field of rubber and treatments employed in that field are applied thereto. Thus handling thereof including vulcanization treatment differs largely from that in the field of fluorine-containing resin. Actually when the elastomer is used for a composition for vulcanization, since a filler, vulcanizing agent, vulcanization accelerator, acid acceptor, etc. are added thereto and since a major application of a rubber material is a sealing material and a contact portion thereof with chemicals is limited to a very small area unlike the chemical pipe, square tank and basket, there have been almost no problem particularly with purity of the fluorine-containing elastomer and presence of impurities.
On the other hand, an attention has been attracted recently to the fluorine-containing elastomer as a sealing material for semiconductor production apparatuses which is used under critical environment in the field of production of semiconductors from the viewpoint of excellent chemical resistance, solvent resistance and heat resistance thereof.
However in the field of production of semiconductors, reduction of cost of semiconductor chips has been advanced rapidly because of high integration and enhancement of yield, and as a result, a sealing material used on semiconductor production apparatuses is strongly required not to contain metal components, TOC and particles and not to release them in the semiconductor production apparatuses (so-called “cleaning” of a sealing material). Therefore it is not enough to employ only a conventional method of washing the sealing material with various solvents.
For example, JP-B-59-46985 discloses a sealing material produced by molding a composition containing a fluorine-containing elastomer by using a peroxide as a vulcanizing agent without adding a metal compound as an acid acceptor, and JP-A-9-500163 discloses a sealing material produced by molding a composition prepared by using an organic acid acceptor without using a metal oxide as an inorganic acid acceptor. However only by those approaches for cleaning during vulcanization, a metal content in the fluorine-containing elastomer used for those sealing materials cannot be reduced sufficiently. Also it has become necessary to take measures against metal ingredients contained in the fluorine-containing elastomer because the sealing materials are used in a plasma atmosphere causing a reduction of weight or in an environment where an abrasion of the sealing material represented by a gate valve arises due to open-close operations.
Further according to a technique described in JP-A-9-500163, O-ring having a reduced metal content is intended to be obtained by extracting a metal component through a specific method after it is produced by molding a composition containing a fluorine-containing elastomer through usual method. However even if such an extra step is employed, since the fluorine-containing elastomer itself is prepared through a conventional method, the content of metal component being present therein is absolutely much and as a result, cannot be reduced sufficiently.
On the other hand, JP-A-8-506604 describes a technique for vulcanizing by irradiating electron beams without using a metal compound. However the vulcanized product still contains a metal component which seems to be attributable to a coagulating agent, in an order of 100 ppm.
Namely a conventional fluorine-containing elastomer may contain metal components substantially because metal compounds such as a polymerization initiator and coagulating agent are used for preparation thereof. Also since metal compounds such as a vulcanizing agent, vulcanizing accelerator, acid acceptor and filler are used for producing a sealing material from such a fluorine-containing elastomer, the sealing material contains metal components and further particles are present on its surface.
Also conventional evaluation of content of metal components is made with an amount of elution under a specific elution (extraction) conditions, and therefore a present situation is such that under different treating conditions, an amount of elution also differs remarkably.
Such being the case, a fluorine-containing elastomer being capable of complying with every environment and treatment and having substantially reduced metal components therein and a sealing material produced from the fluorine-containing elastomer and having a sufficiently reduced content of metal components have not yet been obtained.
In view of the above-mentioned fact, an object of the present invention is to provide a method of substantially reducing a content of metal components inside of the fluorine-containing elastomer in the preparation thereof, and further as a result, to provide a molded article, particularly a sealing material produced from the fluorine-containing elastomer having a sufficiently reduced content of metal components.